PLC scan cycle (input scan, program execution, output update) in PLC Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Order of PLC scan cycle phases

Which of the following correctly lists the phases of a PLC scan cycle in the right order?

A Program execution → Input scan → Output update

B Output update → Input scan → Program execution

C Input scan → Output update → Program execution

D Input scan → Program execution → Output update

💡 Hint

Think about how the PLC reads inputs before running the program and then updates outputs.

The PLC first reads all inputs (input scan), then executes the user program (program execution), and finally updates the outputs (output update). This cycle repeats continuously.

💻 Command Output

intermediate

PLC output state after one scan cycle

Given a PLC program that sets an output coil ON when an input is ON, what will be the output state after one complete scan cycle if the input was ON at the start?

PLC Programming

Input: ON
Program: If input ON then set output coil ON
Initial output coil state: OFF

A Output coil is ON

B Output coil remains OFF

C Output coil state is undefined

D Output coil toggles ON and OFF rapidly

💡 Hint

Remember the PLC reads inputs first, then runs the program logic.

Since the input is ON during the input scan, the program sets the output coil ON during program execution. The output update phase then applies this change, so the output coil is ON after the scan.

🔧 Debug

advanced

Identify the cause of output delay in PLC scan

A PLC output does not change immediately when the input changes. Which part of the scan cycle causes this delay?

A Program execution phase

B Output update phase

C Input scan phase

D Communication phase

💡 Hint

Think about when outputs actually get updated in the cycle.

Outputs are only updated during the output update phase, which happens after program execution. This causes a delay between input change and output response.

🚀 Application

advanced

Effect of long program execution on scan cycle

If the program execution phase takes longer than expected, what is the most likely effect on the PLC scan cycle?

A Overall scan cycle time increases, causing slower response

B Output update will happen before program execution

C PLC will reset automatically

D Input scan will be skipped

💡 Hint

Consider how the scan cycle timing depends on each phase duration.

If program execution takes longer, the entire scan cycle duration increases, which slows down how often inputs are read and outputs updated, causing slower system response.

🧠 Conceptual

expert

Why PLC uses a scan cycle instead of event-driven execution

Why do most PLCs use a continuous scan cycle (input scan, program execution, output update) rather than event-driven execution?

A Because scan cycles allow outputs to update before inputs are read

B Because event-driven execution requires more memory than scan cycles

C Because scan cycles simplify timing and ensure all inputs are checked regularly

D Because event-driven execution cannot handle multiple inputs

💡 Hint

Think about reliability and predictability in industrial control.

Scan cycles provide a predictable, repeatable process that ensures all inputs are read and outputs updated regularly, which is critical for reliable industrial control.

Practice

1. What is the correct order of the PLC scan cycle steps?

easy

A. Output update, input scan, program execution

B. Input scan, program execution, output update

C. Program execution, input scan, output update

D. Program execution, output update, input scan

Solution

1. Step 1: Understand the PLC scan cycle process

   The PLC first reads all inputs to get the current state of sensors and switches.

2. Step 2: Follow the sequence of operations

   After reading inputs, the PLC runs the user program logic, then updates outputs accordingly.

3. Final Answer:

   Input scan, program execution, output update -> Option B

4. Quick Check:

   Scan cycle order = Input, Program, Output [OK]

Hint: Remember: Inputs first, then logic, then outputs [OK]

Common Mistakes:

• Mixing up the order of program execution and input scan
• Thinking outputs update before program runs
• Assuming outputs update before inputs are scanned

2. Which of the following is the correct syntax to represent the PLC scan cycle steps in order?

easy

A. InputScan(); ProgramExecution(); OutputUpdate();

B. OutputUpdate(); InputScan(); ProgramExecution();

C. ProgramExecution(); OutputUpdate(); InputScan();

D. OutputUpdate(); ProgramExecution(); InputScan();

Solution

1. Step 1: Identify the correct function call order

   The PLC scan cycle functions must be called in the order: InputScan, ProgramExecution, then OutputUpdate.

2. Step 2: Match the syntax with the correct order

   InputScan(); ProgramExecution(); OutputUpdate(); correctly shows the functions in the right sequence with proper syntax.

3. Final Answer:

   <code>InputScan(); ProgramExecution(); OutputUpdate();</code> -> Option A

4. Quick Check:

   Function calls order = InputScan, ProgramExecution, OutputUpdate [OK]

Hint: Functions follow the scan cycle order: input, program, output [OK]

Common Mistakes:

• Placing OutputUpdate before InputScan
• Swapping ProgramExecution and OutputUpdate calls
• Incorrect function call syntax

3. Given the following PLC scan cycle pseudo-code, what will be the output state after one full scan if input sensor is ON?

InputScan(): sensor = ON
ProgramExecution(): if sensor == ON then output = ON else output = OFF
OutputUpdate(): set output device to output

medium

A. Output device state will not change

B. Output device will be OFF

C. Output device will be ON

D. Output device will toggle randomly

Solution

1. Step 1: Analyze input scan step

   The sensor input is read as ON during InputScan.

2. Step 2: Evaluate program execution logic

   Since sensor == ON, the program sets output = ON.

3. Step 3: Understand output update effect

   OutputUpdate sets the output device state to ON based on program logic.

4. Final Answer:

   Output device will be ON -> Option C

5. Quick Check:

   Sensor ON leads to output ON [OK]

Hint: Input ON means output ON after program runs [OK]

Common Mistakes:

• Confusing input and output states
• Ignoring program logic conditions
• Assuming output updates before program execution

4. A PLC program is not updating outputs correctly. The code snippet is:

OutputUpdate()
InputScan()
ProgramExecution()

What is the main problem here?

medium

A. OutputUpdate is called before inputs are scanned and program runs

B. InputScan should be last in the cycle

C. ProgramExecution should be before OutputUpdate only

D. No problem, this order is correct

Solution

1. Step 1: Review the correct PLC scan cycle order

   The correct order is InputScan, ProgramExecution, then OutputUpdate.

2. Step 2: Identify the error in the given code

   OutputUpdate is called first, before inputs are scanned and program logic runs, causing incorrect output updates.

3. Final Answer:

   OutputUpdate is called before inputs are scanned and program runs -> Option A

4. Quick Check:

   OutputUpdate must be last in scan cycle [OK]

Hint: Output update must come after input scan and program execution [OK]

Common Mistakes:

• Calling OutputUpdate too early
• Swapping InputScan and ProgramExecution order
• Assuming any order works for scan cycle

5. You want to ensure a safety output turns OFF immediately when an emergency stop input is pressed. Which scan cycle step should you prioritize to minimize delay?

hard

A. None, PLC scan cycle cannot be changed

B. Program execution to run safety logic faster

C. Output update to apply changes immediately

D. Input scan to detect emergency stop as early as possible

Solution

1. Step 1: Understand the scan cycle timing

   The input scan step reads all inputs at the start of the cycle, so detecting emergency stop early is critical.

2. Step 2: Prioritize input scan for safety

   By ensuring input scan happens promptly, the PLC can detect emergency stop quickly and then run program logic to turn off outputs.

3. Step 3: Output update applies changes but depends on earlier steps

   Output update applies the result but cannot act before inputs are scanned and logic runs.

4. Final Answer:

   Input scan to detect emergency stop as early as possible -> Option D

5. Quick Check:

   Early input scan = faster emergency response [OK]

Hint: Detect emergency input ASAP in input scan step [OK]

Common Mistakes:

• Thinking output update speed alone improves safety
• Ignoring input scan timing importance
• Assuming program execution speed is the only factor